The present invention relates to folic acid derivatives useful for radioassaying folic acid and its metabolites in biological fluids such as blood serum. There are many methods known for the radioassay of folic acid and its derivatives in biological fluid. Competitive binding of labeled folic acid (compounds of the present invention) and test sample folic acid for the folate binding protein(s) present in milk (Clinical Chem. 19, 1101, 1973) is a suitable method for determining folic acid.
U.S. Pat. No. 3,989,812 describes compounds of the formula ##STR2## wherein R is .sup.125 I or .sup.131 I substituted in the phenol ring having utility for determining folic acid in serum.
Belgium Pat. No. 840,196 describes tyramine derivatives of folic acid labeled .sup.125 I or .sup.131 I.
The compounds of the present invention are particularly distinct in that the labeled tyrosine moiety is not in the terminal position and that the terminal position is occupied by glutamic acid.
Labeled compounds of the present invention more closely resemble folic acid in that both have glutamic acid in the terminal position.